The present invention relates to a traveling wire electric discharge machining apparatus for machining, e.g., cutting, a workpiece by applying periodic voltage pulses to a wire electrode while it is moved relative to the workpiece.
There have heretofore been known electric discharge wire cutting apparatus, or traveling wire electric discharge machining apparatus, for cutting a workpiece by applying periodic voltage pulses to a wire electrode while moving the wire electrode relative to the workpiece.
A machining process carried out by a conventional electric discharge wire cutting apparatus and problems of such a machining process will be described below.
As shown in FIG. 1A, when a wire electrode 124 cuts a workpiece 102 by traveling along a straight path from a point C to a point D by a distance .delta., the wire electrode 124 is subjected to a force due to a combined vector V1 of electric discharge reactive forces from a new electric discharge machining area S1, in a direction DC.
On the other hand, as shown in FIG. 1B, when the wire electrode 124 is moved along a 90.degree. turn from the point C to a point E by the distance .delta., thus machining a corner into the workpiece 102, a force due to a combined vector V2 of electric discharge reactive forces from a new electric discharge machining area S2 is applied in a direction EF, rather than in a direction EC. As a result, the wire electrode 124 suffers a flexure .alpha. in the direction EF, and is displaced a distance .beta. in a direction perpendicular to the direction EC. Since the wire electrode 124 vibrates owing to its forced displacement, it excessively machines the workpiece 102, causing a corner 113 to be truncated, as shown in FIG. 2.
One conventional system for controlling an electric discharge machining process to reduce the truncating of a corner is disclosed in Japanese patent publication No. 63-2728, for example.
The disclosed electric discharge machining control system detects the amount of variation of the arrival of a wire electrode at an angular area of a shape to be machined in a workpiece, from a programmed positioned. A memory stores a plurality of electric machining conditions in a sequential order, and a controller switches between the stored electric machining conditions stepwise based on amount of variation detected by the detector, until the machined state of the angular area becomes a preset machined state in order to machine the workpiece 102 at a constant speed.
Further, as shown in FIG. 3, the wire electrode 124 is flexed arcuately by a maximum distance .alpha. in a direction opposite to the direction in which the wire electrode 124 machines the workpirce 102. In FIG. 3, the wire electrode 124 is supported by upper and lower wire guides 130, 132, and moves from the left to the right with respect to the workpiece 102. When the arcuately flexed wire electrode 124 changes its direction of movement, it cuts the workpiece 102 along a corner, causing the corner to be truncated.
FIG. 4A shows the path of movement of the wire electrode 124 along an upper surface of the workpiece 102. FIG. 4B shows the path of movement of the wire electrode 124 along the center (measured in the thickness direction) of the workpiece 102. It can be seen from FIGS. 4A and 4B that the corner is more truncated at the center of the workpiece 102, than at the upper surface.
According to the electric discharge machining control system disclosed in the above publication, switching between the electric machining conditions is carried out immediately after the wire electrode 124 turns the corner. Therefore, even with the conventional electric discharge machining control system, the corner is machined by the arcuately flexed wire electrode 124, and it is not possible to sufficiently prevent truncation of the corner.